The availability of pharmaceutical to combat neurological disorders is limited by the existence of the blood-brain barrier (BBB), a protective vascular system preventing most blood-borne molecules from entering the brain. Methods to transport drugs across the BBB would therefore be of enormous therapeutic and commercial value. The long-term goal of this project is to develop an in vitro model for the human BBB to serve as an experimental system for the discovery and evaluation of strategies for drug delivery to the brain. This model will consist of human brain microvascular endothelial cell cultures expressing BBB-associated properties. Phase i feasibility studies, in which these cells were isolated and grown on porous substrata, led to preliminary identification of culture conditions inducing the cell monolayer to restrict solute diffusion. Building upon these data, the development of an in vitro human BBB will continue in Phase II. The first aim will be to define the optimal culture environment for the in vitro human BBB, by screening culture components to identify those that promote the most restrictive cell monolayer. Once one or more culture conditions have been chosen, further studies will characterize the in vitro human BBB by testing it for several properties exhibited by the authentic BBB. These will include BBB-associated proteins, detected with immunological and functional assays, tight junctions, examined by electron microscopy, and specific transport systems that, in vivo, allow the brain to receive essential nutrients. These studies, by addressing the extent to which the in vitro human BBB mimics the BBB in vivo, should facilitate research, in Phase III and beyond, to apply this model as an experimental system for the design and testing of innovative drug delivery systems that circumvent the BBB.